1. Field of the Invention
This invention relates to method and apparatus for assembling a laser module in a telecommunications network and, more particularly, to a thermoelectric cooling element positioned externally of the laser module for maintaining the operating temperature of the laser module.
2. Description of the Prior Art
In a conventional cable television (CATV) system optical transmitters are used to convert radio frequency (RF) signals to optical signals. The conversion is provided by a laser diode which transmits AM modulated or digitally modulated optical signals for communication over a fiber optic CATV distribution network. The laser diode is coupled to a fiber optic network suitable for use in optical transmissions.
Laser diode devices are sensitive to operating conditions such as temperature, modulating signal level, and the loss or improper application of current which biases the laser diode to its stimulated or laser emission state. As the ambient temperature of the optical transmitter increases or decreases, unequal thermal expansion of the components creates stresses on the components which can alter their optical characteristics. For example, the optical beam emitted from the laser diode is focused to a modulator. Thermal stresses applied to the laser diode misalign the optical beam, resulting in a reduction in the output of the optical transmitter. Therefore, the laser diode must be maintained at a stable operating temperature, i.e. within a range of plus or minus two degrees.
In a conventional optical communications system, the laser diode and the other optical components in the system are buried in underground conduits and the like, exposing them to extreme environmental conditions. The optical elements may be exposed to ambient temperatures ranging from xe2x88x9230xc2x0 C. to about 50xc2x0 C. Because the oscillation characteristics of a semiconductor laser diode have a large temperature dependency, if the module is not maintained at a constant temperature. shifts in the threshold current density or oscillation wavelength may take place depending on temperature variation.
In order to accommodate the temperature variations to which the laser diode is exposed and to avoid wavelength shifts, it is the conventional practice to hold the operating temperature of the laser diode by a Peltier-element electronic cooling device. By keeping the operating temperature of the laser diode at a constant temperature, the operating bias and, therefore, the total optical transmission of the optical signal is maintained at a constant level under extreme environmental conditions and in response to electrical disturbances.
An example of the prior art semiconductor laser module using an electronic cooling device in an optical communication system is disclosed in U.S. Pat. No. 6,181,718 in what is commonly referred to as a butterfly-type module package. A semiconductor laser diode is mounted on a carrier which is coupled internally to an electronic cooling device. The cooling device includes a Peltier-element having a pin junction sandwiched between a first dielectric plate substrate and a second dielectric plate substrate. To avoid a temperature increase in the diode, a temperature detector, such as a thermistor-resistor, is installed around the diode. With this arrangement, the temperature around the diode is maintained by the supply current to the Peltier-element. A change in the temperature is detected by the thermistor and in response actuates an increase or decrease of the electric current to the Peltier-element. The current is increased to increase the rate of heat flow from the diode and thereby cool the diode back to the operating temperature. When the temperature measured by the thermistor is lower than the operating temperature, the electric current to the Peltier-element is decreased to decrease the rate of heat flow from the laser diode. The heat generated in the diode raises the temperature thereof back to the operating temperature.
With the above-described butterfly-type module construction, the thermoelectric cooling element is mounted internally within the package in contact with the laser diode. The package is formed by a hermetically sealed metal-ceramic or metal-glass rectangular package with multiple leads protruding from opposite sides of the package. External electronic circuitry is used to control the operation of the internal cooling element. However, this configuration adds significantly to the cost of manufacture of the laser diode module.
Another example of a laser diode module having an internally packaged thermoelectric cooler (TEC) is disclosed in U.S. Pat. No. 5,181,214. All the elements of the laser diode module are mounted to a common, temperature stabilized base plate. The base plate is fabricated of a low thermal expansion material, such as copper-tungsten alloy. The base plate is mounted on a thermoelectric cooler, which in turn is mounted on a heat sink. The thermoelectric cooler controls the rate of heat flow between the base plate and the heat sink in order to maintain the temperature of the laser diode at a predetermined operating temperature.
In U.S. Pat. No. 5,379,145, an optical transmitter for light wave communications utilizes a thermistor thermally coupled with a laser diode in a module. Also, thermally coupled to the laser diode is a thermoelectric cooler. The TEC is connected to a controller responsive to voltage developed across the thermistor to turn current to the TEC on and off to cool the laser diode when its temperature exceeds a certain temperature. The other components are within the package.
In the multichannel analog optical fiber communication system disclosed in U.S. Pat. No. 5,034,334, the laser diode chip is mounted on a metallized carrier. The carrier is in turn attached to a copper stud cooled by a conventional thermoelectric cooling element to maintain the laser diode at about 20xc2x0 C.
Conventional integrated optical transmitter in a CATV system includes an optical head assembly generating a formed optical beam and an optical modulator which receives the formed optical beam for modulation. An optical head assembly is maintained in a fixed relationship by an epoxy bonding to the modulator. The optical head includes a laser diode that is coupled to the modulator for transmitting an optical beam to the modulator. A thermal transfer plug couples a rear portion of the optical head assembly to a TEC to transfer heat therebetween. A second TEC is coupled by adhesive directly to the optical head. TECs are conventionally operable to remove or add heat from the modulator and optical head assembly to maintain optimum operating temperature. Further, it is disclosed that a thermistor is mounted in the transfer plug to monitor the temperature of the optical head assembly. All these components are contained in an integrated package.
Another example of a butterfly type module package for a semiconductor laser diode in an optical fiber telecommunications system is disclosed in U.S. Pat. No. 6,219,364. A laser diode chip and a thermistor are mounted via a heat sink on a submount. The submount is in turn mounted on a metal substrate. The metal substrate is bonded by a hard metal solder to the top of a Peltier-element. The Peltier-element is in turn sandwiched by ceramic panels so that the cooler element is internally mounted within the module beneath the laser diode.
Another example of a thermoelectric cooling element mounted internally within the laser diode package is disclosed in U.S. Pat. No. 6,018,536. An integrated laser package includes a gain element supported on a high thermal conductive submount in alignment with a fiber of an optical coupling means which is also supported on the submount. The submount is in turn supported on a TEC cooler. Thus, all the elements are heat sunk to the same support in an integrated package and maintained at the same temperature.
One disadvantage of an integrated laser module where the thermoelectric cooling element is contained within the module is the high manufacturing cost of the integrated design. Internally mounting the TEC element requires a support structure for positioning the laser diode on top of the TEC element. This requires hermetically sealing the TEC element in a metal-ceramic or metal-glass rectangular package and soldering the package to a substrate for supporting the laser diode above the TEC element. The manufacturing cost of a laser diode module would be substantially reduced by fabricating the module with the TEC element positioned externally of the module. This would eliminate many of the components required to support the TEC element internally in heat transfer relation with the laser diode. Therefore, there is need in a laser diode module for mounting the thermoelectric cooling element externally of the module to reduce the manufacturing cost of the module.
In accordance with the present invention, there is provided a laser diode assembly in a telecommunications network that includes a module of a preselected configuration housing the laser diode. A thermoelectric cooling element has opposing surfaces. A first of the opposing surfaces is a controlled surface maintained at a selected operating temperature. A second of the opposing surfaces is an uncontrolled surface for transfer of heat to and from the cooling element. A heat sink is positioned in thermal contact with the cooling element second surface. The module has a heat transfer surface positioned in spaced relation oppositely of the cooling element first surface. A thermally conductive material fills the space between the module heat transfer surface and the cooling element first surface. The thermally conductive material conforms to the surface configurations of the module and the cooling element to place the module in thermal contact with the cooling element for transfer of heat therebetween. A measuring device is supported by the conductive material for detecting a change in the temperature of the cooling element first surface from the operating temperature. The temperature applied to the cooling element is adjusted to transfer heat between the module, the cooling element, and the heat sink to maintain the cooling element at the operating temperature.
Further in accordance with the present invention there is provided a method for assembling a laser diode in a telecommunications network that includes the steps of housing a laser diode within a laser module having an outer surface fabricated of a thermally conductive material. The laser module is connected to a telecommunications network for the transmission of optical signals thereto. A thermoelectric cooling element is positioned oppositely of the laser module to transfer heat therebetween. The cooling element is positioned in thermal contact with a heat sink for transfer of heat between the cooling element and the heat sink. A thermally conductive material is inserted between and in contact with opposing surfaces of the laser module and the cooling element to place the laser module and the cooling element in thermal contact for the transfer of heat therebetween. The temperature of the cooling element is measured to detect a change in the temperature of the laser module from a preselected temperature in response to heat transferred from the laser module to the cooling element. The electric current supplied to the cooling element is adjusted to transfer heat between the laser module and the heat sink through the cooling element to maintain a preselected temperature of the laser module.
In addition the present invention is directed to apparatus for controlling the temperature of a semiconductor laser diode that includes a module housing the laser diode in thermal contact therewith. The module has a thermally conductive surface of a preselected configuration. A thermoelectric cooling element is positioned in heat transfer relation with the module. The cooling element has a thermally conductive surface of a preselected configuration. A heat sink is positioned in thermal contact with the thermoelectric cooling element. A thermally conductive filler material is positioned between the module and the cooling element in conformity with the configurations of the thermally conductive surfaces of the module and the cooling element to place the module and the cooling element in thermal contact for the transfer of heat between the laser diode and the heat sink.
Accordingly, a principal object of the present invention is to provide method and apparatus for maintaining the operating temperature of a semiconductor laser diode in a module or package by a cooling element independently controlled and positioned externally of the module and maintained in thermal conductivity with the module for a transfer of heat therebetween.
Another object of the present invention is to provide apparatus for housing a semiconductor laser diode in a package free of the electrical components associated with a temperature controller housed externally of the package.
Another object of the present invention is to provide method and apparatus for positioning a thermoelectric cooling element externally of a housing for a laser module where the adjacent surfaces of the cooling element and laser module are placed in thermal conductivity by a thermally conductive filler material conforming to the adjacent surfaces.
Another object of the present invention is to provide a package for housing a semiconductor laser diode used in a telecommunications network where the manufacturing cost of the package is reduced by using a TEC element externally of the package.
A further object of the present invention, is to provide a package for a semiconductor laser module in a telecommunications network having a temperature control device positioned externally in thermal contact with the module.
These and other objects of the present invention will be more completely disclosed and described in the following specification, the accompanying drawings, and the appended claims.